U.S. Pat. No. 4,628,136 (the '136 patent) teaches a method of recovering the heat contained in the overhead of the ethylbenzene/styrene monomer (EB/SM) splitter by using this stream to boil an azeotropic mixture of ethylbenzene and water, which, once vaporized, is subsequently transferred to the reaction system where dehydrogenation of ethylbenzene to styrene takes place. As described in the '136 patent, the EB feed is vaporized with water in the overhead of the EB/SM separation Column. This is possible as EB and water forms a low boiling point azeotrope.
Referring now to FIG. 1, a simplified process flow diagram for the azeotropic heat recovery similar to that as described in the '136 patent is illustrated. Crude styrene from the dehydrogenation reactor (or upstream separations) is fed via flow line 10 to the EB/SM splitter 12. Styrene product is recovered as a bottoms fraction 14, and ethylbenzene, possibly along with other impurities such as benzene, toluene, and xylenes (BTX), are recovered as an overheads fraction 16. The overheads fraction 16 is condensed via indirect heat exchange with ethylbenzene (recycle and/or fresh) and water (such as condensate recovered from the dehydrogenation product), fed via flow line 18, in azeotropic vaporizer 20. The condensed overhead fraction is recovered from azeotropic vaporizer 20 via flow line 22, a portion of which may be used for column reflux, and a portion of which may be fed to downstream processes (not shown), such as for the recovery of BTX when these components are not separated upstream of the EB/SM splitter. The vaporized azeotropic mixture of EB and water is recovered from azeotropic vaporizer 20 via flow line 24 for feed to the dehydrogenation reaction zone (not illustrated).
The weight ratio of EB and water vapor in stream 24 is commonly referred to as the Primary Steam to Oil weight ratio in the dehydrogenation reaction area. (PS/Oil weight ratio). This configuration, as described in the '136 patent, saves the energy associated with the boiling of EB and water as this mixture is vaporized against EB/SM Separation column overhead vapor, which would otherwise be condensed using cooling water.
Referring now to FIG. 2, a simplified flow diagram for a typical configuration for the dehydration reaction area is illustrated. SM is manufactured by dehydrogenating the EB feed, which is an endothermic reaction. The vaporized azeotropic mixture of EB and water is fed via flow line 24 to the reaction zone, which may include two to four dehydrogenation reactors 26, 28. The effluent from each reactor 26 may be reheated using steam before entering the next reactor 26 or final reactor 28. The steam used for reheating the reactor effluents is commonly referred to as Main Steam (MS), which is provided from a steam superheater 30 via flow line 32 and eventually enters at the inlet 34 of the first reactor 26 along with the PS/Oil (vaporized EB/water) mixture, which may also be preheated against the effluent from final reactor 28 in exchanger 36.
As noted in the background of the '136 patent, the focus in the industry may fluctuate periodically between energy efficiency and catalyst developments, among other concerns. However, improvements in these distinct areas may affect the overall process. For example, new catalysts are available, and others may be in development, which allow operation of the dehydrogenation reactor at lower overall steam to oil weight ratios ((MS+PS)/oil). For example, new catalysts being developed may allow for operation at an overall steam to oil weight ratio of 0.9 to 1.0, or even lower.
The azeotropic vaporization of the ethylbenzene-water mixture, at conditions suitable for cross-exchange with the overheads from the EB/SM splitter, provides only a limited variability in the control of the PS/Oil weight ratio of the vaporized azeotropic mixture. As a result, operation at lower overall steam to oil weight ratios would require a decrease in the amount of main steam (MS). However, decreasing the amount of main steam impacts the reheating of reactor effluents between the reaction stages. Thus, with a smaller amount of MS, higher furnace and transfer line temperatures are required as the same reaction heat needs to be provided (for equivalent SM production rates). However, at overall S/O weight ratios of 1.0 or lower, the temperatures needed to provide the required heat may exceed the current metallurgical limitations of the heater coils 38 as well as the associated transfer lines.